A process in which information is embedded in material for the purpose of identifying the material is referred to as watermarking.
Identification code words are applied to versions of material items for the purpose of identifying the version of the material item. Watermarking can provide, therefore, a facility for identifying a recipient of a particular version of the material. As such, if the material is copied or used in a way, which is inconsistent with the wishes of the distributor of the material, the distributor can identify the material version from the identification code word and take appropriate action.
In this description, an item of material, which is copied or used in a way, which is inconsistent with the wishes of the originator, owner, creator or distributor of the material, will be referred to for convenience as an offending item of material or offending material.
The material could be any of video, audio, audio/video material, software programs, digital documents or any type of information bearing material.
For a watermarking scheme to be successful, it should be as difficult as possible for the users to collude in order to mask or remove the identification code words. It should also be as difficult as possible for users to collude to alter the identification code word to the effect that one of the other users is falsely indicated as the perpetrator of an offending item of material. Such an attempt by users to collude to either mask the code word or alter the code word to indicate another user is known as a collusion attack.
Any watermarking scheme should be arranged to make it difficult for users receiving copies of the same material to launch a successful collusion attack. A watermarking scheme should therefore with high probability identify a marked material item, which has been the subject of a collusion attack. This is achieved by identifying a code word recovered from the offending material. Conversely, there should be a low probability of not detecting a code word when a code word is present (false negative probability). In addition the probability of falsely detecting a user as guilty of taking part in a collusion attack, when this user has not taken part, should be as low as possible (false positive probability).
U.S. Pat. No. 5,664,018 discloses a watermarking process in which a plurality of copies of material items are marked with a digital watermark formed from a code word having a predetermined number of coefficients. The watermarked material item is for example an image. The apparatus for introducing the watermark transforms the image into the Discrete Cosine Transform (DCT) domain. The digital watermark is formed from a set of randomly distributed coefficients having a normal distribution. In the DCT domain each code word coefficient is added to a corresponding one of the DCT coefficients. The watermarked image is formed by performing an inverse DCT. A related publication entitled “Resistance of Digital Watermarks to Collusion Attacks”, by J. Kilian, F. T. Leighton et al, published by MIT, Jul. 27, 1998, provides a detailed mathematical analysis of this watermarking process to prove its resistance to attack.